Declaring "I have come to speak to you about how the war in Iraq will end," President Barack Obama today fulfilled the defining promise of his campaign, announcing that all U.S. combat troops will be withdrawn by September 2010.

But in the same speech, he revealed that the vast majority of those involved in the pullout will not leave this year. He also said that tens of thousands of U.S. personnel will remain behind afterward.

"The most important decisions that have to be made about Iraq's future must now be made by Iraqis," Obama said before hundreds of Marines and off9icers at the sprawling Camp Lejeune, N.C., base, which is deploying thousands of troops to the other war front in Afghanistan.

Senior Obama administration officials said that of the roughly 100,000 U.S. combat troops to be pulled out of Iraq over the next 18 months, most will remain in the war zone through at least the end of this year to ensure national elections there go smoothly. The pace of withdrawal suggests that although Obama's promised pullout will start soon, it will be backloaded, with most troops returning toward the end of the time frame.

And even with the drawdown, a sizable U.S. force of 35,000 to 50,000 U.S. troops will stay in Iraq under a new mission of training, civilian protection and counterterrorism.

The Aug. 31, 2010, the end date for Iraq war combat operations is slower than Obama had promised voters as a candidate. The timetable he pledged then would have seen combat end in May 2010.

Regardless, it is a hastened exit, something Obama called a necessity, both for the future of Iraq and to allow the U.S. to refocus its attention more firmly on Afghanistan.

"We cannot rid Iraq of all who oppose America or sympathize with our adversaries," he said. "We cannot police Iraq's streets until they are completely safe, nor stay until Iraq's union is perfected. We cannot sustain indefinitely a commitment that has put a strain on our military, and will cost the American people nearly a trillion dollars."